x11/qt5/qtdeclarative: build with python 3

Builds (and packages without PLIST change) without python%2.7 installed.
I'm doing this to cut down the output of
`show-reverse-deps lang/python/2.7' which lists a bunch of X tools that
all pull old Python through this (which is only a build dependency, but
show-reverse-deps(1) does not distinguish between build and runtime).

OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/x11/qt5/qtdeclarative/Makefile,v
retrieving revision 1.16
diff -u -p -r1.16 Makefile
--- Makefile 23 Feb 2021 19:39:52 -0000 1.16
+++ Makefile 28 Feb 2021 12:16:08 -0000
@@ -23,7 +23,6 @@ WANTLIB-main += GL Qt5Core Qt5Gui Qt5Ne
WANTLIB-main += c m pthread

MODULES = lang/python
-MODPY_VERSION = ${MODPY_DEFAULT_VERSION_2}
MODPY_RUNDEP = No

LIB_DEPENDS-examples = ${LIB_DEPENDS} \